The effect of mastoparan, a tetradecapeptide peptide isolated from wasp venom, on the release of arachidonic from egg yolk lecithin liposomes, rat peritoneal mast cells and cultured human fibroblasts was studied. Mastoparan caused a dose-dependent increase in the production of arachidonic acid catalyzed by phospholipase A2 from bee venom, rattlesnake and porcine pancreas. Mastoparan also stimulated phospholipase activity in tissue homogenates and caused a dose-dependent release of arachidonic acid from mast cells and fibroblasts. The results suggest that mastoparan and related peptides found in hymenoptera venom act, at least in part, by stimulating phospholipase activity. A new screening test: release of amylase from dispersed guinea pig pancreatic acinar cells was set up. Using this test, two new peptides were isolated from venom sac extracts of Vespa crabro. Peptide 1, Leu-Asn-Leu-Lys-Ala-Leu-Leu-Ala-Val-Lys-Lys-Ile-LeuNH2, has been named mastoparan C because it has the properties of mastoparan. Peptide 2, Phe-Leu-Pro-Leu-Ile-Leu-Arg-Lys-Ile-Val-Thr-Ala-LeuNH2, has not been given a trivival name because its biological properties are still under investigation. It has appreciable activity in releasing histamine from mast cells. Using immunocytochemical techniques, ranatensin has been localized in rat brain. Numerous perikarya and terminals throughout the brain were immunostained but the highest concentration of immunoreactive ranatensin, by far, was found in the pineal gland and the neurointermediate lobe of the pituitary gland. Bradykinin and other kinins reverse the inhibitory effect of morphine, metenkaphalin and epinephrine on the electrically estimulated guinea pig ileum longitudinal muscle-myenteric plexus preparation by enhancing the release of acetylcholine. Human urine contains immunoreactive granuliberin suggesting that humans contain a hitherto unrecognized peptide structurally related to granuliberin which is histamine-releasing peptide isolated from frog skin. The peptide in Gila monster venom which actively releases amylase from panacreatic ancinar cells has been further characterized.